1. Field of the Invention
The present invention relates to a gas sensor using a semiconductor material (a semiconductor gas sensor) and a technique for producing the same, and particularly relates to a semiconductor gas sensor, which is suitable for detecting hydrogen gas, has high reliability and high sensitivity, and can be applied also to an operation in a high temperature environment, and an effective technique to be applied to the production thereof.
2. Background Art
As a gas sensor to be used for detecting hydrogen gas, there are provided a Si-MOSFET (a metal-oxide-semiconductor field-effect transistor) type gas sensor having a Pd (palladium) gate structure (e.g., I. Lundstrom et al., Applied Physics Letters, Vol. 26, No. 2, 15 January, 55-57 (1975) (NPL 1)), and a Si-MOSFET-type gas sensor using Pt (platinum) or another metal in the platinum group as a gate metal. However, such a gas sensor has a problem of lack of reliability (e.g., Introduction of T. Usagawa et al., Journal of Applied Physics, Vol. 108, 074909 (2010) (NPL 2)). Therefore, in the case of a Pt gate structure, in order to prevent the lack of reliability and the contamination of a device in a process line due to film peeling, it is necessary to insert a metal film made of Ti (titanium), Mo (molybdenum), or the like as an adhesive film between a Pt film and a gate insulating film (e.g., a SiO2 (silicon oxide) film). However, the insertion of such an adhesive film caused a problem that the gas sensor does not respond to hydrogen.
Therefore, in order to solve the problem, there has been developed a MISFET (metal-insulator-semiconductor field-effect transistor) type gas sensor having a Pt—Ti—O (platinum-titanium-oxygen) gate structure. This MISFET-type gas sensor having a Pt—Ti—O gate structure has been disclosed in, for example, JP-A-2009-300297 (PTL 1), T. Usagawa et al., Sensors and Actuators, B160, 105-114 (2011) (NPL 3), and the above-described NPL 2.
The Pt—Ti—O gate structure has an oxygen-doped Ti film (hereinafter also referred to as a “modified Ti film”) formed by mixing oxygen-doped amorphous Ti and amorphous TiOx (titanium oxide) or a TiOx nanocrystal on a gate insulating film (e.g., a SiO2 film), and also has a Pt film on this oxygen-doped Ti film. The Pt film is composed of a plurality of Pt crystal grains, and in a grain boundary region existing among the plurality of Pt crystal grains, Ti and oxygen (O) are present (hereinafter also referred to as a “modified Pt film”).
However, in the above-described MISFET-type gas sensor having a Pt—Ti—O gate structure in the related art, the rising response time is several tens to several hundreds of seconds in some cases, and therefore, measures for such a hydrogen response characteristic are needed.
On the other hand, there is a need for a gas sensor to be used in a harsh outside environment, for example, in a containment of a diesel automobile or a nuclear power plant, to detect exhaust gas (e.g., ammonia) or hydrogen gas. In this case, there is a demand for the use of a gas sensor in a serious disaster in which a nuclear power plant or the like is largely destroyed for some reasons or in a high-temperature (e.g., from 300° C. to 900° C.) gas environment in a turbine, a diesel internal-combustion engine, or the like.
However, when the above-described MISFET-type gas sensor having a Pt—Ti—O gate structure in the related art is operated for several tens of days at a temperature of from about 300° C. to 400° C., a phenomenon in which the rising response time is delayed to several hundreds of seconds was observed.
As a hydrogen gas sensor aiming at the operation thereof at a high temperature (e.g., 800° C.), for example, a SiC-MOSFET-type gas sensor using SiC (silicon carbide) (e.g., R. Loloee et al., Sensors and Actuators, B129, 200-210 (2008) (NPL 4)) has been studied.
However, a sensor signal drifts or an unidentified signal of as much as about 0.6 V is detected even at a low concentration (52.2 ppm), and a basic technique for a gate electrode, a passivation film, a source-drain electrode, a heater, etc. capable of operating stably at a high temperature for a long period of time has not been established yet. In particular, other than the problem of reliability attributed to crystallinity, the fact that a gate electrode having high reliability has not been found yet is the biggest obstacle to putting it into practical use.
For example, the biggest obstacle to obtaining reliability of a Pt gate structure is as follows. When the thickness of a Pt film is 100 nm, a void with a size of several micrometers is formed by annealing at 800° C. for several hours in some cases, and when the thickness of a Pt film is 300 nm, a void with a size of several micrometers or a crack is formed by annealing at 700° C. in some cases (e.g., A. Branzahi et al., Sensors and Actuators, B26/27, 165-169 (1995) (NPL 5)). Due to this, a problem arises in sensor signal drift at a high temperature and reproducibility.